perf(pipeline): MP-Alloc Task 4 - reuse per-frame animatedIds + drawableCells sets

Completes the safe-batch (implementer finished this but died before
committing; coordinator independently verified bit-identity + safety
post-interruption). Two per-frame HashSet<uint> allocations replaced by
reused cleared-in-place fields:
- GameWindow._animatedIdsScratch: WalkEntitiesInto treats null and empty
  identically (line 682 `is null || Count == 0`), so an always-non-null
  empty set == the old null-when-empty local. Verified.
- RetailPViewRenderer._drawableCellsScratch: flows into
  RetailPViewFrameResult.DrawableCells (live ref) but every consumer reads
  it same-frame; sigDrawableCells is a per-frame OnRender local that
  EmitRenderSignatureIfChanged only FORMATS to a string (no reference
  retention, no _lastSig set field). Built frame N -> read frame N ->
  cleared frame N+1. No cross-frame corruption.

Build green, full suite 4116 passed / 4 skipped.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/AcDream.App/Rendering/RetailPViewRenderer.cs

@@ -50,6 +50,13 @@ public sealed class RetailPViewRenderer
     // visible.
     private readonly InteriorEntityPartition.Result _partitionResult = new();
 
+    // MP-Alloc (2026-07-05): DrawInside's drawable-cell set, reused across
+    // frames instead of `new HashSet<uint>(pvFrame.OrderedVisibleCells)` every
+    // call. Every consumer (DrawEntityBucket, DrawExitPortalMasks,
+    // DrawCellObjectLists, RetailPViewFrameResult.DrawableCells) reads it
+    // synchronously within the same frame it was built.
+    private readonly HashSet<uint> _drawableCellsScratch = new();
+
     // T2 (BR-4): retail has NO distance constant on the flood-admission chain
     // (DrawBuilding → portal walk → ConstructView: viewconeCheck + side test +
     // GetClip + GetVisible only). The old 48 m seed cap is replaced by the
@@ -118,7 +125,9 @@ public sealed class RetailPViewRenderer
         // so every visible cell's shell has a prepared batch and seals — killing the grey
         // (the old clipAssembly.CellIdToSlot.Keys filter silently dropped slot-less cells).
         // Per-slice trim still applies in DrawEnvCellShells (Task 4 makes it self-contained).
-        var drawableCells = new HashSet<uint>(pvFrame.OrderedVisibleCells);
+        _drawableCellsScratch.Clear();
+        _drawableCellsScratch.UnionWith(pvFrame.OrderedVisibleCells);
+        var drawableCells = _drawableCellsScratch;
         UseIndoorMembershipOnlyRouting();
 
         // #124: look-in cells need prepared shell batches + their statics routed